Demand for secondary batteries as an energy source has been significantly increased as technology development and demand with respect to mobile devices have increased. Among these secondary batteries, lithium secondary batteries having high energy density, high voltage, long cycle life, and low self-discharging rate have been commercialized and widely used.
A lithium secondary battery denotes a battery in which a non-aqueous electrolyte containing lithium ions is included in an electrode assembly which has a microporous separator disposed between a cathode including a cathode active material capable of intercalating and deintercalating lithium ions and an anode including an anode active material capable of intercalating and deintercalating lithium ions.
For example, as a cathode active material of a lithium secondary battery, transition metal oxides, such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), or lithium nickel oxide (LiNiO2), or composite oxides having a portion of the above transition metals substituted with other transition metals have been used.
Among the above cathode active materials, since LiCoO2 has excellent various properties such as cycle characteristics, LiCoO2 has currently been widely used. However, LiCoO2 has low stability, is expensive as a raw material due to the resource limit of cobalt, and has limitations in being mass used as a power source of applications such as electric vehicles.
Since lithium manganese oxides, such as LiMnO2 or LiMn2O4, have advantages in that resources thereof are abundant as a raw material and environmentally-friendly manganese is used therein, lithium manganese oxides have receive great attention as a cathode active material that may replace LiCoO2. However, these lithium manganese oxides have disadvantages in that their capacity is small and cycle characteristics are poor.
Lithium has been initially used as an anode active material constituting an anode of a lithium secondary battery. However, since lithium may have low reversibility and safety, a carbon-based active material has currently been mainly used as the anode active material of the lithium secondary battery. Although the carbon material may have a lower capacity than lithium, the carbon material may have smaller volume changes as well as excellent reversibility and may also be advantageous in terms of cost.
The carbon-based active material may be categorized into an amorphous carbon-based active material and a crystalline carbon-based active material such as graphite. The amorphous carbon-based active material may have high discharge capacity and excellent rate characteristics, but may have disadvantages in that irreversible capacity is high, charge and discharge efficiency is poor, and energy density is poor due to low volume density and electrical conductivity. In contrast, the crystalline carbon-based active material has low discharge capacity, but the crystalline carbon-based active material has very good energy density, has good potential flatness, and has relatively better reversibility between charge and discharge processes than the amorphous carbon-based compound.
Thus, it is important to select an active material layer that may improve the performance of the battery in consideration of the above characteristics. In general, with respect to electrodes, charge balance between the cathode and the anode must be well maintained, and various problems may occur when the charge balance is not maintained because output characteristics of any one of the cathode and the anode become much better. That is, in the case that the output characteristics of the cathode are better than those of the anode, since the insertion and release of lithium are maximized, limitations due to the insertion and release of a large amount of lithium may occur. Thus, life characteristics of the lithium secondary battery may be reduced. In contrast, in the case in which the output characteristics of the anode are better than those of the cathode, lithium is not intercalated into the cathode and a side reaction may occur.